1. Field of the Invention
The present invention relates generally to a receiver. More particularly, the present invention relates to a direct conversion receiver, which directly down-converts a received radio-frequency (RF) signal.
2. Description of the Related Art
Heterodyne receivers were used as receivers in cellular phones over the past years. However, recently, there has been a rise in the use of direct conversion receivers. In the direct conversion receiver, RF signals are converted directly into baseband signals, whereby separate intermediate frequency stages are not required.
FIG. 1 is a block diagram illustrating a construction of a conventional direct conversion receiver. As illustrated in FIG. 1, the conventional direct conversion receiver includes a low-noise amplifier (LNA) 100, a band pass filter (BPF) 102, first and second frequency mixers 110 and 106, a 90 degree phase shifter 104, and first and second low pass filters (LPF) 112 and 108. RF signals from an antenna (not shown) are directed to the LNA 100. The RF signals are amplified by the LNA 100. The amplified RF signals from the LNA 100 are band-pass filtered by the BPF 102. An output signal of the BPF 102 is divided into an in-phase (I) channel path for an in-phase (I) component and a quadrature-phase (Q) channel path for a quadrature-phase (Q) component. The in-phase (I) component and quadrature phase (Q) component are supplied to the first and second frequency mixers 110 and 106, respectively. A local frequency signal cos ωLOt is directed to the first frequency mixer 110. Also, a phase of the local frequency signal cos ωLOt is shifted by 90° by means of the 90 degree phase shifter 104, and the phase-shifted local frequency signal is directed to the second mixer 106. The first mixer 110 multiplies the in-phase (I) component from the BPF 102 by the local frequency signal cos ωLO t. The second mixer 106 multiplies the quadrature phase (Q) component from the BFP 102 by the 90° phase-shifted local frequency signal. The first LPF 112 low-pass filters an output of the first mixer 110 and outputs a down-converted in-phase signal I. The second LPF 108 low-pass filters an output of the second mixer 106 and outputs a down-converted quadrature phase signal Q. As described above, RF signals are down-converted directly into baseband signals, whereby separate intermediate frequency stages are not required.
However, as shown in FIG. 3A, in the conventional direct conversion receiver, a direct current offset component is included in outputs of the first and second mixers 110 and 106, namely, the in-phase signal I and the quadrature phase signal Q. FIG. 3A illustrates a simulation result of the conventional direct conversion receiver by means of a Hewlett Packard Advanced Design System. A code division multiple access (CDMA) signal is used as a simulation target signal. A transverse axis of FIG. 3A indicates a frequency, and the unit of the frequency is MHz. A longitudinal axis of FIG. 3A indicates a power, and the unit of the power is dBm. When analog in-phase signal I and quadrature phase signal Q are converted into digital signals by an analog-digital converter (ADC), the DC offset component included therein narrows the dynamic range of a received CDMA signal in a cellular phone.